Direct velocity seismic sensing

ABSTRACT

A disclosed direct velocity seismic sensor includes a housing, a proof mass suspended in the housing by a resilient component, and a motion dampener that damps oscillation of the proof mass to a degree that displacement of the proof mass relative to the housing is substantially linearly proportional to a rate of change of seismic displacements of the housing over a frequency range of interest. A described method for constructing a seismic sensor includes using a calculated resonant frequency to determine a damping factor that causes the displacement of the proof mass to be substantially proportional to the rate of change of seismic displacement of the housing. One illustrative disclosed system includes an optical velocity sensor and a detector where a light beam produced by the velocity sensor and a reference beam interfere at the detector, and the detector produces a signal indicative of a velocity experienced by the velocity sensor.

CROSS-REFERENCE TO RELATED APPLICATIONS

The present application incorporates by reference and claims priority asa continuation-in-part to U.S. patent application Ser. No. 12/655,111,titled “Direct Velocity Seismic Sensing” and filed Dec. 23, 2009, byinventor Robert Fernihough.

BACKGROUND

Marine seismic explorations usually employ seismic sensors below thewater's surface, e.g., in the form of streamers towed behind a ship orcables resting on the ocean floor. A typical seismic streamer includesmultiple sensors positioned at spaced intervals along its length, andoften many such streamers or cables positioned in parallel lines overthe survey region.

An underwater seismic wave source, such as an air gun, produces pressurewaves that travel through the water and into the underlying earth. Whensuch waves encounter changes in acoustic impedance (e.g., at a boundaryor layer between strata), a portion of the wave is reflected. The wavesreflected from subsurface layers are called “seismic reflections”. Theseismic streamers or cables provide an array of seismic sensors todetect these seismic reflections and convert them into signals forstorage and processing.

One notable consequence of operating in the marine environment is thepresence of “ghost reflections” caused by pressure wave reflections offthe water's surface. The downward-moving ghost reflections can interferewith the sensors' measurements of the upward-moving seismic reflections,causing substantial amplitude enhancements at some frequencies (due toconstructive interference), and reductions at other frequencies (due todestructive interference).

To address this issue, the industry developed the usage of dual sensorsat each sensing node. A pressure sensor (“hydrophone”) and a velocitysensor (“geophone”) provide measurements of pressure and (directional)velocity that, when appropriately combined, enable ghost reflections tobe filtered out of the survey data. Such techniques are standard in theindustry. Accordingly, seismic explorationists have come to expect bothtypes of sensors to be available when specifying parameters foracquiring seismic survey data.

There exists certain technologies that offer potential advantages forconducting long-term seismic monitoring and/or seismic data acquisitionin extreme marine environments. As one example, efforts have been madeto develop optical seismic sensors that demonstrate high reliability,have long lifetimes, and do not require any electrical power. Suchresults have resulted in the creation of optical hydrophones andaccelerometers, but to date the author is aware of no satisfactoryoptical geophones.

SUMMARY

Accordingly, there is disclosed herein a technique for direct velocityseismic sensing, along with various sensors and methods that employ thistechnique. One sensor embodiment includes a housing, a proof masssuspended in the housing by a resilient component, and a motion dampenerthat damps oscillation of the proof mass to a degree that displacementof the proof mass relative to the housing is proportional to a rate ofchange of seismic displacements of the housing over a frequency range ofinterest. A described method for constructing a seismic sensor includesusing a calculated resonant frequency to determine a damping factor thatcauses the displacement of the proof mass to be substantiallyproportional to the rate of change of seismic displacement of thehousing. One illustrative disclosed system includes an optical velocitysensor and a detector where a light beam produced by the velocity sensorand a reference beam interfere at the detector, and the detectorproduces a signal indicative of a velocity experienced by the velocitysensor.

BRIEF DESCRIPTION OF THE DRAWINGS

A better understanding of the various disclosed embodiments can beobtained when the following detailed description is considered inconjunction with the following drawings, in which:

FIG. 1 shows an illustrative marine seismic survey being carried outabout an offshore oil platform;

FIGS. 2A-D show different embodiments of a seismic data acquisitionsystem;

FIGS. 3A-C show different embodiments of a direct velocity sensor;

FIG. 4 shows the magnitude and phase of a proof mass's velocity responseversus frequency for several values of a damping factor ζ; and

FIG. 5 is a flowchart of an illustrative method for constructing adirect velocity optical seismic sensor.

While the invention is susceptible to various modifications andalternative forms, specific embodiments thereof are shown by way ofexample in the drawings and will herein be described in detail. Itshould be understood, however, that the drawings and detaileddescription thereto are not intended to limit the invention to theparticular form disclosed, but on the contrary, the intention is tocover all modifications, equivalents and alternatives falling within thespirit and scope of the appended claims.

DETAILED DESCRIPTION

The issues identified in the background are at least partly addressed bythe direct velocity sensing technique disclosed herein. Various opticalsensors and optical sensing methods that employ this sensing techniqueare disclosed, and they can be used to provide seismic sensing arrayssuitable for permanent monitoring of a subsurface reservoir. Otherapplications of such sensors and sensing methods include seismicmonitoring in remote or extreme environments. Though the followingdescription is given in the context of permanent reservoir monitoring,the disclosed technology is readily adaptable for use in conjunctionwith conventional seismic data acquisition systems, e.g., forexploration survey sensing with towed marine streamers or ocean-bottomsensor arrays.

FIG. 1 shows a marine seismic survey being carried out about an offshoreoil platform 100 located in an ocean 102. Several wells 104 extenddownward from the offshore oil platform 100, through the water 102, andinto the subsurface 106 below the ocean floor. A seismic sensor system108 includes an array access port 110, an umbilical cable 112, anaggregator box 114, one or more ocean floor cables 116 extendingoutwardly from the aggregator box 114, multiple sensor housings 118positioned along the ocean floor cables 116, and one or more sensorspositioned within each of the sensor housings 118.

It is expected that in many cases, the array of seismic sensors will beleft permanently in place to enable repeat surveys of the subsurfaceregion around platform 100. (Such repeat surveys enable operators totrack movement of reservoir fluids and optimize their exploitation ofthe subsurface reservoirs.) Port 110 provides surface access to theseismic sensors via optical fibers in the umbilical cable 112 and in theocean floor cables 116. Operators on platform 100 can connect equipmentto the access port 110 to collect seismic survey data as describedfurther below.

The umbilical cable 112 extends from the array access port 110 to theaggregator box 114. In the embodiment of FIG. 1, the aggregator box 114is located near a base of the offshore oil platform 100, and on theocean floor 126. The ocean floor cables 116 extend from the aggregatorbox 114 in a pattern that provides an arrangement of regularly spacedseismic sensor housings 118. The use of optical fibers in cables 116,118 is expected to enable these cables to be extremely rugged andreliable.

In FIG. 1, a ship 120 is towing a seismic source 122 producing acousticwaves. The seismic source 122 may be or include, for example, an airgun. An acoustic wave 124 produced by the seismic source 122 is shown inFIG. 1. As indicated in FIG. 1, the acoustic wave 124 travels throughthe water and into the subsurface 106, generating seismic reflections ateach of the acoustic impedance changes that it encounters. For clarity,only one such reflection is shown from interface 128.) The array ofseismic sensors is positioned to detect many of these reflections as theseismic source is fired at intervals over a set of selected shotlocations.

Each of the sensor housings 118 along the ocean floor cable 116preferably includes multiple sensors such as an optical hydrophone andan optical geophone. Multiple geophones can be provided to permitmulti-axial velocity sensing. To prevent movement due to ocean currentsor erosion, the sensor housings 118 can be coupled to the sea floor in arobust manner (e.g., via high-density weights or spikes driven into theocean floor), permitting the location and orientation of the sensors tobe precisely determined and documented. Because the system 108 employsoptical sensors driven by light via optical fibers, it requires nounderwater electronic components or electrical power, therebyeliminating problems from short circuits, corrosion in electricalconnectors, and the like.

FIG. 2A is a diagram of one embodiment of a seismic data acquisitionsystem 200. In the embodiment of FIG. 2A, the seismic data acquisitionsystem 200 includes: one or more coherent light source(s) 202, a mirror204, an optional optical delay 206, a beam splitter 208, and the seismicsensor system 108 of FIG. 1, represented in FIG. 2A by the array accessport 110, one or more cable(s) 210, and sensors 212. Cables 210represent the cables of the seismic sensor system 108, i.e., theumbilical cable 112 and the ocean floor cable 116, and sensors 212represent the sensors located in each of the sensor housings 118 ofFIG. 1. Also shown is a surface processing facility 226 that includesone or more detector(s) 214, a signal processor 216, data recording andprocessing circuitry 218, a memory 220 storing position information andother parameters, a general purpose data processing system 222, and amap storage and/or display system 224.

As described above, the array access port 110 provides access to thecables of the seismic sensor system 108 of FIG. 1 (i.e., the umbilicalcable 112 and the ocean floor cable 116). The light source(s) 202, themirror 204, the optional optical delay 206, the beam splitter 208, andthe detector(s) 214 are included in equipment (e.g., a portable unit)that couples to the array access port 110 to optically drive sensors 212and derive signals therefrom. The signal processor 216, recorder 218,and memory 220 can be included in the portable module or provided aspart of a separate module that connects to the portable module todigitize and store seismic survey data in the form of seismic traceshaving position information and other associated parameters. The generalpurpose data processing system 222 and map storage/display 224 can alsobe included in the on-site equipment connected to the access port 110,but in many cases the seismic survey data collected by recorder 218 istransported to a central processing facility having substantialcomputational resources available.

In the embodiment of FIG. 2A, the sensors 212 are optical sensorsrequiring no electrical power to operate. The coherent light source(s)202 may be or include, for example, gaseous lasers, solid-state lasers,and semiconductor-based lasers. The light sources 202 produce a beam oflight that is received by the beam splitter 208. The beam splitter 208provides a transmitted portion of the light beam to the sensors 212 viathe array access port 110 and the cables 210, and a reflected portion ofthe light beam to an optional optical delay 206. The beam passingthrough the optical delay 206 is reflected off a fixed mirror 204 tocreate a reference beam that is returned to the beam splitter 208.

Light passing through the optional optical delay 206, from one side toan opposite side, is delayed due to travel time. Such delays can beprovided by a number of mechanisms, though a spool of optical fiber isoften the most practical method. In some embodiments, the optical delaytime of the optional optical delay 206 is substantially equal to half ofa delay time experienced by the transmitted portion of the light beam intraveling from the beam splitter 208 to the sensors 212. Thus, the lightthat traverses the delay twice experiences roughly the same travel timeas the light that passes through the sensors 212. With light sourceshaving extremely long coherence times, the optical delay can be omittedwithout adverse effect.

Each of the sensors 212 receives the transmitted portion of the lightbeam from the beam splitter 208 via the access port 110 and the cables210, and modifies the received light beam in response to a measuredquantity (e.g., pressure, temperature, acceleration, velocity, etc.).This modified light beam has at least one characteristic (e.g.,amplitude and/or phase) indicative of the measured quantity. Thismodified (“measurement”) beam is returned to the beam splitter 208 viathe cables 210 and the array access port 110.

The beam splitter 208 provides the detectors 214 with a combined beamthat includes both the reference beam and the measurement beam. Thereference beam and measurement beam interfere with each other, causingthe detectors to sense a light intensity that varies based on the pathlength difference between the reference and measurement beams. Forexample, if the path length difference is some integer number ofwavelengths, the beams interfere constructively to produce increasedlight intensity. Conversely, if the path lengths differ by an oddmultiple of a half wavelength, the beams interfere destructively toproduce decreased light intensity. The detectors are accordingly able tosense changes in the path length difference as cycles in the intensityof the light.

The coherent light provided to the sensors 212 is multiplexed so thatthe measurement beams produced by the various sensors 212 can bedifferentiated from one another. For example, the coherent lightprovided to the sensors 212 may be wavelength (frequency) multiplexedsuch that each of the sensors 212 receives coherent light within adifferent range of wavelengths (frequencies). To enable suchmultiplexing, the coherent light source 202 should generate a broadbandbeam, possibly by using multiple sources each producing light in adifferent band. Conversely, the detector 214 can include multipledetectors, each designed to respond to a different one of the multipleranges of wavelengths (frequencies).

Alternatively, the coherent light returned from the sensors 212 can betime division multiplexed such that each of the sensors 212 receiveslight within the same range of wavelengths (frequencies), but returns ameasurement beam at different times. With time division multiplexing,the coherent light source(s) 202 may include a single source producingcoherent light within a single range of wavelengths (frequencies).Different periods of time would be associated with each of the sensors212, and the measurements made by detector 214 at those times areassociated with the corresponding sensor.

Signal processor 216 may be or include, for example, an analog todigital converter that receives the analog signals produced by thedetector(s) 214, and produces digital data corresponding to the analogsignals. The data recording and processing circuitry 218 is a dataacquisition system with a interface enabling a user to program andcontrol the acquisition process using a computer system such as a laptopcomputer or a desktop computer. The data recording and processingcircuitry 218 receives the digital data produced by the signal processor216, and accesses the memory 220 to retrieve the position informationand other parameters corresponding to each of the sensors 212. Theacquisition system also collects position information for the seismicshots or at least time-stamp information that enables the correct shotlocations to be determined later. The acquisition system 218 combinesthe digital data form the signal processor 216 with the positioninformation and other parameters to obtain and store seismic traces. Theseismic survey data collected in this manner is then made available tothe general-purpose data processing system 222.

The general-purpose data processing system 222 may be or include, forexample, a personal computer, an engineering workstation, a mainframecomputer, or the like. The general-purpose data processing system 222performs one or more seismic processing operations on the seismic surveydata to construct a model of the subsurface in the survey region,thereby producing seismic attribute maps, images, and/or otherinformation for users. Such information can be displayed via a displaysystem 224 and/or stored for later use. The map storage and/or displaysystem 224 may include, for example, a data storage device for storingthe image information, and/or a computer monitor for displaying theimage information.

FIG. 2B shows an alternative embodiment of a portion of the seismic dataacquisition system 200. In this embodiment, a beam splitter 250, opticaldelay 206, and a mirror 204 are positioned near the sensors 212, e.g.,in the aggregation box 114 or one in each sensor housing 118. The beamsplitter 250 splits the coherent light beam into a reference beam and ameasurement beam as described above, and further recombines the beams asdescribed above. However, because these components are positioned at thefar end of cables 210, the optical delay 206 can be much shorter.Moreover, the attenuation experienced by each beam will be bettermatched.

FIG. 2C shows another alternative embodiment of a portion of the seismicdata acquisition system 200. In this embodiment, the light to thesensors travels a separate path than the light returned from thesensors. Separate optical fibers are provided in cables 210 fordown-going and up-going light beams. Within each sensor housing 118, aband-limited splitter 252 extracts a selected band of light from theincoming light fiber, routes the extracted band to a splitter 254 thatcreates a measurement beam and a reference beam. The measurement beampasses through sensors 260, while the reference beam passes through anoptical delay 206. The two beams are recombined by a second splitter 256and inserted in beam on the outgoing light fiber by a secondband-limited splitter 258. As with the embodiment of FIG. 2B, theoptical delay element is quite small and the attenuation of the twobeams is automatically matched. However, the component count is higher.

FIG. 2D shows a separate-path embodiment in which the reference beam isgenerated at the surface by splitter 270 and recombined with themeasurement beams by splitter 278. In this embodiment, each sensorhousing 118 only requires the optical sensors and the band limitedsplitters 272, 274. (As before, band-limited splitter 272 extracts aselected band from the downgoing fiber and band-limited splitter 274inserts the modified beam onto the upgoing fiber.) Furtherimplementation details can be found in U.S. Pat. No. 6,850,461“Fiber-Optic Seismic Array Telemetry, System, and Method” by S. J. Maaset al.

FIG. 3A is a diagram of one embodiment of a direct velocity sensor 300.One or more of the sensors 212 of the seismic data acquisition system200 of FIGS. 2A-D may be or include such a direct velocity sensor 300.In the embodiment of FIG. 3A, the direct velocity sensor 300 includes ahousing 304, a proof mass 306, a resilient component 310, a motiondampener 308, and a terminator 302 for an optical fiber 312. In theembodiment of FIG. 3A, the proof mass 306, the motion dampener 308, andthe resilient component 310, are positioned within the housing 304. Aterminator 302 mounts the optical cable 312 to an upper wall of thehousing 304.

The terminator 302 receives coherent light (e.g., laser light) from thecable 312, and directs the coherent light, labeled 314 in FIG. 3A,toward an upper surface of the proof mass 306. The upper surface of theproof mass 306 is adapted to reflect a substantial portion of theincident coherent light 314. That is, the upper surface of the proofmass 306 is preferably highly reflective with respect to the incidentcoherent light 314. The coherent light 314 from the terminator 302strikes the upper surface of the proof mass 306 and reflects back towardthe terminator 302 as modified coherent light 316. The terminator 302receives the modified coherent light 316 from the upper surface of theproof mass 306 and directs the modified coherent light 316 to the cable312.

The proof mass 306 is suspended within the housing 304 by the resilientcomponent 310, represented in FIG. 3A by a coil spring. Resilientcomponent 310 provides a restoring force that operates to return theproof mass 306 to its original position once external excitations cease.The resilient component 310 may be or include, for example, one or moresprings such as coil springs and/or leaf springs. It can also be acompressible liquid or gas. The resilient component 310 may also be orinclude, for example, one or more elements formed of a resilientmaterial such as rubber or foam. Alternatively, the resilient component310 may be or include a cantilever beam, such as a cantilever beam of amicro-electromechanical system (MEMS) device. In some embodiments, theresilient component is coupled to the proof mass by a substantiallyincompressible fluid (i.e., by a hydraulic mechanism).

In the embodiment of FIG. 3A, the resilient component 310 is shownpositioned between the proof mass 306 and a lower wall of the housing304 opposite the upper wall through which the terminator 302 extends. Inother embodiments, the resilient component may be positioned between theproof mass 306 and the upper wall of the housing 304, or between theproof mass 306 and both the upper and lower walls of the housing 304.(Note that in the descriptions of these figures the terms “upper” and“lower” refer to positions in the figure and do not imply any particularorientation of the sensor.)

The motion dampener 308 damps movement of the proof mass 306. In FIG.3A, the motion dampener 308 is represented by a dashpot including apiston that moves within a cylinder containing a fluid (e.g. air, water,oil, or the like). In other embodiments, the motion dampener 308 may beor include other mechanisms using a fluid to damp the movement of theproof mass 306. For example, a fluid may be located between the proofmass 306 and the housing 304 such movement of the proof mass 306relative to the housing 304 is inelastically resisted (i.e., damped) bythe fluid. Further, the housing 104 may be substantially filled with afluid such that movement of the proof mass 306 within the housing 304 isdamped by the fluid.

Alternatively, the motion dampener 308 may be or include a passiveelectrical circuit that dissipates energy when the proof mass 306 movesrelative to the housing 304. Alternatively, the motion dampener canemploy active damping, in which energy is added to counter the motion ofthe proof mass. Active damping can be provided in a number of ways, suchas using motion of a secondary proof mass to generate sympatheticcurrents that induce magnetic fields to counter the motion of theprimary proof mass. Another way to provide active damping employs afeedback circuit to generate a drive signal that at least partiallycounters motion of the proof mass 306 within the housing 104. In yetanother alternative motion dampener embodiment, the resilient componentitself performs double-duty as a motion dampener. For example, resilientmaterials such as rubber or foam often dissipate energy in the form ofheat when they are compressed or stretched.

In the embodiment of FIG. 3A, the direct velocity sensor 300 has asensing axis 318 that passes substantially through a center of thedirect velocity sensor 300 and is substantially perpendicular to theupper and lower walls of the housing 304. When the housing 304experiences a change in velocity with respect to time (i.e., anacceleration) along the sensing axis 318, the momentum of the proof mass306 delays movement of the proof mass 306 relative to the housing 304,causing relative motion to occur between the proof mass 306 and thehousing 304 along axis 318. A distance ‘D1’ between the upper surface ofthe proof mass 306 and the terminator 302 either increases or decreases,depending on a direction of the acceleration. As a result of therelative motion between the proof mass 306 and the housing 304, a lengthof a path that the coherent light travels within the housing 304 eitherincreases or decreases. As a result of the change in path length, aphase of the modified coherent light 316 differs relative to the phaseof the reference light beam, enabling a detector at the surface tomeasure motion of the sensor 300.

As described above, the motion dampener 308 damps the movement of theproof mass 306 within the housing 304. As described in more detailbelow, a damping factor provided by the motion dampener 308 is selectedsuch the displacement D1 is proportional to the velocity of the housing304 over a frequency range of interest. More specifically, when thesensor is subjected to oscillatory motion in a specified frequencyrange, the displacement of the proof mass is linearly proportional tothe sensor's velocity so long as the sensor features the appropriatecombination of mass, spring constant, and damping.

The proof mass 306, the resilient component 310, and the motion dampener308 form a mechanical system that converts motion of the housing intorelative motion of the proof mass. It can be shown that the ratiobetween the relative displacement of the proof mass and the displacementof the housing is given by:

${{X(f)} = \frac{f^{2}}{\left( {f^{2} - {j \cdot 2 \cdot \zeta \cdot f_{n} \cdot f} - f_{n}^{2}} \right)}},$where f is the frequency of the input motion, f_(n) is the natural orresonant frequency of the system, ζ is the damping coefficient providedby the motion dampener 308, and j is √{square root over (−1)}. (Thenatural frequency of a mechanical system is

${f_{n} = {\frac{1}{2\pi}\sqrt{\frac{k}{m}}}},$where k is the spring constant and m is the oscillating mass.)

In a similar fashion, the ratio between the relative displacement of theproof mass and the velocity of the housing can be shown to be:

${{V(f)} = \frac{j \cdot f}{\left( {2 \cdot \pi} \right) \cdot \left( {f_{n}^{2} + {j \cdot 2 \cdot \zeta \cdot f_{n} \cdot f} - f^{2}} \right)}},$and the ratio between the relative displacement of the proof mass andthe acceleration of the housing can be shown to be:

${A(f)} = {\frac{1}{\left( {4 \cdot \pi^{2}} \right) \cdot \left( {f_{n}^{2} + {j \cdot 2 \cdot \zeta \cdot f_{n} \cdot f} - f^{2}} \right)}.}$

Since our interest here is to construct an optical geophone, we selectthe second equation above for more detailed analysis. FIG. 4 shows themagnitude of V(f), i.e., the displacement of the proof mass 306 relativeto a velocity of the housing 304. Both axes are logarithmic, with thevertical axis having units of decibels (dB) and the horizontal axisrepresenting frequency on a logarithmic scale. Also shown is the phaseof V(f) (in degrees) versus frequency on a logarithmic scale. In bothgraphs, the natural frequency is shown for reference.

Of particular interest is the shape of the V(f) magnitude curve. As thedamping factor increases, the curve levels out over a substantialfrequency range. For ζ≧2, there is a range of frequencies about thenatural or resonant frequency f_(n), for which the displacement of theproof mass 306 is substantially directly proportional to the velocity ofthe housing 304, and this range of frequencies increases for increasingvalues of the damping factor ζ. At the same time, the V(f) phase curveshows that for ζ≧2, the phase is substantially linear (or at leastchanges gradually) over the range of frequencies for which thedisplacement of the proof mass 306 is substantially directlyproportional to the velocity of the housing 304. Such gradual rates ofchange in phase are highly desirable in sensor systems. In addition, thesecond graph also shows that the rate of change of the phase of V(t)becomes more gradual for increasing values of the damping factor ζ. Theinventor takes these characteristics as suggesting that with theappropriate damping factor, proof mass displacement sensing can serve asa superior direct velocity measurement technique that avoids any noiseenhancement penalties that would be inherent in sensors having avariable sensitivity to velocity.

The V(f) magnitude curve shows that it is possible to select a dampingfactor ζ, provided by the motion dampener 308, that achieves a desiredfrequency range over which the displacement of the proof mass 306 issubstantially directly proportional to the velocity of the housing 304.(The selection process for the damping factor ζ is described in moredetail below.) Accordingly, the damping factor ζ provided by the motiondampener 308 is preferably selected dependent upon a desired frequencyrange of interest, and the selected damping factor ζ is typicallygreater than 2.

As described above, one or more of the sensors 212 of the seismic dataacquisition system 200 of FIG. 2 may be or include the direct velocitysensor 300. The light passing through optical fiber 312 may be split offfrom a fiber in one of the cables 210, and the modified coherent light316 may form one of the modified light beams provided to the beamsplitter 208 via the cables 210 and the array access port 110.

FIG. 3B shows an alternative embodiment of the direct velocity sensor300. In the embodiment of FIG. 3B, a second optical cable 330 enters thehousing 304 through the lower wall of the housing 304, and terminates ata terminator 332 mounted to the proof mass 306. A portion of thecoherent light 314 produced by the terminator 302 is received by theterminator 332.

In the embodiment of FIG. 3B, when the housing 304 experiences a changein velocity with respect to time (i.e., an acceleration) along thesensing axis 318, a distance ‘D2’ between the terminators 302 and 332either increases or decreases, depending on a direction of theacceleration. As a result of relative motion between the proof mass 306and the housing 304, a length of the path that the coherent lighttravels within the housing 304 either increases or decreases, causing achange in the path length of the measurement beam relative to thereference beam. The damping factor provided by the motion dampener 308is again selected to make the displacement D2 proportional to thevelocity of the housing 304 over a frequency range of interest.

FIG. 3C shows another alternative embodiment of the direct velocitysensor 300. In the embodiment of FIG. 3C, the direct velocity sensor 300includes a magnet 350 mounted within the housing 304 such that themagnet 350 moves with the housing 304. The proof mass 306 is, orincludes, a coil of wire 352 having an axis parallel to the sensing axis318, and having two ends. A portion of the coil of wire 352 is shown inFIG. 3C. An electrically resistive element 354 is electrically connectedbetween the two ends of the coil of wire 352. As described in moredetail below, the coil of wire 352 and the electrically resistiveelement 354 form a motion damper.

In the embodiment of FIG. 3C, the proof mass 306 is coupled to the upperwall of the housing 304 by the resilient component 310. The cable 312enters the housing 304 through the upper wall and is connected to theproof mass by terminator 302. The terminator 302 directs the coherentlight 314 toward an inner surface (e.g., the lower wall) of the housing304. The inner surface is adapted to reflect a substantial portion ofthe incident coherent light 314. That is, the inner surface of the lowerwall of the housing 304 is preferably highly reflective with respect tothe incident coherent light 314. The coherent light 314 from theterminator 302 strikes the inner surface of the lower wall of thehousing 304, and is reflected back toward the terminator 302 as themodified coherent light 316. The terminator 302 receives the modifiedcoherent light 316 from the inner surface of the lower wall of thehousing 304, and provides the modified coherent light 316 to the cable312.

In the embodiment of FIG. 3C, when the housing 304 experiences anacceleration along the sensing axis 318, the momentum of the proof mass306 delays the movement of the proof mass 306 relative to the housing304, causing relative motion to occur between the proof mass 306 and thehousing 304. A distance ‘D3’ between the terminator 302 and the innersurface of the lower wall of the housing 304 either increases ordecreases, depending on a direction of the acceleration. As withprevious embodiments, this displacement is made proportional to thevelocity of the housing.

As described above, the coil of wire 352 and the electrically resistiveelement 354 form a motion damper. In the embodiment of FIG. 3C, as thehousing 304 moves relative to the proof mass 306, the magnet 350(coupled to the housing 304) moves relative to the coil of wire 352,inducing a current in the wire. The resistive element dissipates some ofthe electrical energy as heat, thereby damping motion of the proof mass.The resistance of the element 354, together with the resistance of thecoil, is set to provide the desired damping factor.

FIG. 5 is a flowchart of one embodiment of a method 500 for constructinga seismic sensor (e.g., the direct velocity sensor 300 of FIGS. 3A-C).During a first step 502 of the method 500, a minimum frequency and amaximum frequency of a frequency range of interest are selected. Typicalfrequency ranges of interest for seismic sensing are 2-250 Hz, 1-500 Hz,3-3000 Hz, and 6-8000 Hz. The frequency range of interest extends fromthe minimum frequency to the maximum frequency, and in step 504 thenatural or resonant frequency of the system is chosen to be the neargeometric mean of the minimum and maximum frequencies. That is, if theminimum frequency is termed ‘f_(L)’ and the maximum frequency is termed‘f_(H)’, the ideal value for the natural or resonant frequency f_(n) isf_(n)=√{square root over (f_(L)·f_(H))}.

To implement a sensor with this natural frequency, the proof mass andresilient component spring constant are chosen in an appropriate ratio.When the mass of the sensor housing is much greater than the proof mass,the ratio of proof mass m and spring constant k can be chosen using theequation

$f_{n} = {\frac{1}{2\pi}{\sqrt{\frac{k}{m}}.}}$In block 506, the proof mass is connected to the sensor housing with aresilient component that enables the proof mass to oscillate at thenatural or resonant frequency f_(n).

In block 508, a dampening factor is determined. With reference to FIG.4, it can be observed that the shape of the V(f) magnitude curve isdetermined by the dampening factor, and it is desired to provide amagnitude curve that is substantially flat over the frequency range ofinterest. In some embodiments, the system designer chooses a roll-off atthe minimum and maximum frequencies f_(L) and f_(H). Thus, for example,the roll-off might be chosen to be 3 dB, meaning that at the minimum andmaximum frequencies, the magnitude curve has dropped to 1/√{square rootover (2)} times the magnitude at the natural frequency. Morespecifically:|V(f _(L))|=0.7071·|V(f _(n))|, and|V(f _(H))|=0.7071·|V(f _(n))|.Using these equations, the appropriate value for the dampening factor ζcan be found using standard equation solving techniques. For example, a3 dB roll off at the boundaries of a 2-250 Hz sensor is obtainable witha damping factor of ζ=5.5. Note that the dampening factor need not belimited to real valued numbers, but can be extended to complex valuednumbers. In terms of implementation, complex dampening factors can beprovided using electronic dampening with complex impedances rather thanresistive elements. Such impedances employ capacitive or inductiveelements in addition to dissipative resistors.

During a step 510, a motion dampener (e.g., the motion dampener 308 ofFIGS. 3A-3B or 354 in FIG. 3C) is coupled to the proof mass, where themotion dampener provides the determined damping factor ζ. In step 512, amechanism is provided for optically sensing the displacement of theproof mass relative to the housing. For example, one or more opticalfibers can be mounted to the housing and/or proof mass to direct ameasurement beam across the gap between the housing and the proof massand parallel to the sensing axis. A secondary mechanism can be providedfor recombining the measurement beam with a reference beam, either inthe sensor housing or remote from the sensor housing. The resultinginterference fringes can be monitored as previously described todetermine the displacement of the proof mass relative to the housing.

When the housing is subjected to seismic waves in the frequency range ofinterest, the proof mass oscillates with a relative displacement equalto the rate of change of the seismic waves (subject to a relativelyconstant scale factor that changes only gradually with frequency). Theoscillation of the proof mass is tracked by equipment that monitorsphase changes in a light beam traversing the gap between the proof massand the sensor housing. Those of ordinary skill in the art understandthe complexities of extracting a position signal in a system thatemploys coherent interference with a reference beam, so such issues arenot addressed further here.

The inventor has observed that existing optical seismic sensing systemsmay suffer from certain potential vulnerabilities that may be resolvedwith the disclosed direct velocity sensors, making these systems farmore robust, versatile, and affordable. In existing systems, seismicaccelerometers tend to be designed with underdamped parasitic resonancesto achieve adequate sensitivity and substantially linear performance inthe frequency range of interest. The parasitic resonances may bepositioned far outside the frequency range of interest and would in mostcases be of little concern due to the availability of electronicband-limiting filters. However, equivalent optical band-limiting filtersare generally far more difficult to implement, making optical seismicsystems vulnerable to certain weaknesses.

Optical seismic systems generally employ optical interferometry toremotely probe an array of seismic accelerometers and return an opticalsignal to receiver electronics for digitization and processing. Careshould be taken to minimize the chance that the parasitic resonancesmight cause aliasing during the digitization process. In the case ofsystems employing wavelength and/or frequency multiplexing toconcurrently collect measurements from multiple sensors, care shouldalso be taken to minimize the chance that the parasitic resonances mightcause crosstalk between sensors. To increase resistance to such effects,designers can employ implementation tradeoffs. For example, designerscan increase digitization rates to fight aliasing, can widen theseparation between frequency channels to fight crosstalk, and can employmore expensive band limiting optical filters to better attenuate theeffects of parasitic resonances.

However, such tradeoffs may be made with certain assumptions regardingthese parasitic resonances, which assumptions necessarily impose limitson the manner in which the sensor array can be employed. Thus, thetradeoffs chosen for a system design having an on-bottom cable in deepwater may assume an incident energy level far lower than would be thecase for a typical on-bottom cable in shallow water (less than about 100feet deep) or for a towed streamer. Because the source(s) are generallypositioned more closely to the sensing array in the latterconfigurations, these configurations tend to be far more likely toencounter excitation of parasitic resonances, and hence would typicallyrequire a sensing array design far more resistant to parasitic resonanceeffects. Conversely, the tradeoffs employed for a sensing array designedfor towed or shallow water cable configurations might unnecessarilywaste bandwidth and drive up costs of a deep water system.

The disclosed direct velocity sensors require no such parasiticresonances, and hence they may enable optical seismic system designersto avoid the difficulties created by such parasitic resonances. As canbe seen in FIG. 4, the response of the disclosed direct velocity sensorsis band-limited and essentially immune to parasitic resonances. Concernsabout aliasing and crosstalk are sharply reduced when employing opticalgeophones in a wavelength and/or frequency multiplexed system, enablingthe use of lower digitization rates, smaller and more closely spacedfrequency channels, and/or band limiting optical filters with loosertolerances. Such systems would also tend to be more tolerant ofdifferent usage environments, enabling the same system design to beemployed for deep water, shallow water, and towing surveys. Oneillustrative direct velocity optical sensor for an on-bottom cablesystem design was given a resonance frequency of 44.72 Hz and a dampingfactor of 2.1 (thereby providing a substantially flat response in thevelocity domain between lower and upper roll-off frequencies of 10 Hzand 200 Hz, respectively). When compared with existing systems, the newdesign would be expected to reduce the required digitization rate by afactor of about 3.7, while simultaneously improving signal-to-noiseratio due to a significantly better response at low frequencies.

Other optical techniques for monitoring oscillation of a proof mass areknown and can be employed. See, e.g., U.S. Pat. No. 5,134,882 (Taylor),U.S. Pat. No. 5,903,349 (Vohra), and U.S. Pat. No. 6,921,894(Swierkowski) which employ strain sensing in optical fibers. Moreover,the direct velocity sensing techniques disclosed herein can be used inconjunction with any suitable position sensing method. The capacitanceof an electrically conductive proof mass relative to a housing wall canbe monitored to determine the displacement of the proof mass. Halleffect sensors can be used to monitor the displacement of a magnet onthe proof mass or the housing. The resistance change induced by strainin a thin wire can be used to monitor relative displacement of the proofmass. Travel times of acoustic pulses can be used to monitor relativedisplacement of the proof mass. Numerous other variations andmodifications will become apparent to those skilled in the art once theabove disclosure is fully appreciated. For example, the disclosed directvelocity sensing technique is expected to have application outside theseismic sensing arena. It is intended that the following claims beinterpreted to embrace all such variations and modifications.

What is claimed is:
 1. A marine geophysical survey method thatcomprises: deploying one or more cables or streamers in a marineenvironment, each cable or streamer having frequency- orwavelength-multiplexed optical geophones positioned at spaced intervals;collecting velocity measurements from the optical geophones, wherein thecollecting includes: optically measuring in each geophone a displacementof a resiliently-coupled proof mass relative to a sensor housing, theproof mass being sufficiently damped to make the displacementproportional to a velocity of the housing over a frequency range ofinterest; and imaging a survey region based at least in part on thevelocity measurements.
 2. The method of claim 1, wherein said deployingcomprises at least one of: towing one or more steamers having saidoptical geophones while collecting velocity measurements; and restingone or more cables on bottom in shallow water, said cables having saidoptical geophones.
 3. The method of claim 1, further comprisingrepeating said collecting and imaging to track movement of reservoirfluids.
 4. The method of claim 1, wherein each said cable or streamerfurther includes hydrophones positioned at spaced intervals, and whereinsaid imaging the survey region is also based in part on measurementsfrom the hydrophones.
 5. The method of claim 1, wherein said collectingincludes combining a light beam from each optical geophone with areference beam.
 6. The method of claim 1, wherein said collectingfurther includes employing wavelength and/or frequency multiplexing toconcurrently obtain the velocity measurements from multiple opticalgeophones.
 7. The method of claim 1, wherein said collecting includesreceiving, with a terminator, light that has traveled between the proofmass and the housing.
 8. A marine geophysical survey system thatcomprises: at least one seismic source; a seismic sensing array havingone or more cables or streamers deployed in a marine environment, eachcable or streamer having frequency- or wavelength-multiplexed opticalgeophones positioned at spaced intervals, wherein each optical geophonecomprises: a housing; and a proof mass coupled to the housing by aresilient component, wherein motion of the proof mass is sufficientlydamped to make displacement of the proof mass relative to the housingproportional to a velocity of the housing over a frequency range ofinterest; and a processing facility that optically measures thedisplacement of each proof mass to collect and store velocitymeasurements in response to firings of the at least one seismic source.9. The system of claim 8, wherein the seismic sensing array comprises atleast one towed marine streamer or at least one ocean-bottom cable. 10.The system of claim 9, wherein the seismic sensing array is permanentlydeployed for monitoring a subsurface reservoir.
 11. The system of claim8, wherein the seismic sensing array includes at least one cable onbottom in shallow water, said cable having said optical geophones. 12.The system of claim 8, wherein each cable or streamer in the seismicsensing array further includes hydrophones positioned at spacedintervals.
 13. The system of claim 8, wherein each optical geophonefurther comprises a terminator adapted to receive light that hastraveled between the proof mass and the housing.
 14. The system of claim13, wherein the terminator is coupled to the housing and adapted toreceive light reflected from a surface the proof mass.
 15. The system ofclaim 13, wherein the terminator is coupled to the proof mass andadapted to receive light reflected from a surface the housing.
 16. Thesystem of claim 13, wherein the terminator is coupled to the housing,and wherein each optical geophone further comprises another terminatorcoupled to the proof mass and adapted to direct light toward thehousing-coupled terminator.
 17. The system of claim 13, wherein thedamping is provided by a motion dampener comprising a coil and aresistive element.
 18. The system of claim 13, wherein the damping isprovided by a motion dampener comprising a fluid.
 19. The system ofclaim 13, wherein the damping is provided by a motion dampener thatemploys active damping.